1. Field of the Invention
Priority is claimed to Japan Pat. App. Ser. No. 2010-079292, filed Mar. 30, 2010, hereby incorporated by reference in its entirety.
The present invention relates to a waveguide-type polarizer and more particularly, to a waveguide-type polarizer in which an optical waveguide having a ridge structure is formed on a Z-cut lithium niobate substrate.
2. Description of Related Art
In the technical field of optical communication or optical information processing, an optical waveguide element having an electro-optical effect, such as a lithium niobate (LN) substrate, has been used. For example, there is an LN optical modulator in which a waveguide has a Mach-Zehnder (MZ) structure. In the modulator having the MZ structure, light is turned on or off by a voltage applied. Specifically, it is possible to turn light on or off on the basis of whether an output waveguide of the modulator having the MZ structure is in the single mode, or the fundamental mode and the exited mode in which the propagation speed of light changes depending on the voltage applied.
The characteristics of the LN optical modulator include Vπ and an optical bandwidth. Vπ is the voltage required to change light from an on state to an off state, and the optical bandwidth is a frequency response in which light is turned on or off. As Vπ is reduced and the optical bandwidth increases, the characteristics of the LN optical modulator are improved.
In order to reduce Vπ, it is necessary to reduce the thickness of a buffer layer and thus reduce the distance between an electrode and light. In order to widen the optical bandwidth, it is necessary to increase the thickness of the buffer layer such that a current is not concentrated on one point of the electrode. A predetermined reciprocal relationship is established between Vπ and the optical bandwidth. In order to reduce Vπ and widen the optical bandwidth, a ridge structure is used for the Z-cut LN substrate.
In recent years, a modulator that modulates intensity and a phase, such as a tandem optical modulator, has been mainly used. In order to control the intensity and phase, it is necessary to arrange a plurality of optical waveguides having the MZ structure. In particular, a DQPSK modulator using the Z-cut LN substrate requires six electrodes. Therefore, the modulation electrodes occupy most of the area of the surface of an LN optical modulator chip, which makes it difficult to integrate members for providing additional characteristics.
Since LN has anisotropy in the electro-optical effect, light polarized in a specific direction is incident on the LN optical modulator. In the LN optical modulator, in general, extraordinary light (ne) is polarized as incident light. However, since a commercial Ti-diffused optical waveguide having a ridge structure guides both extraordinary light (ne) and ordinary light (no), it is necessary to remove the ordinary light (no).
For example, as a method of forming an extraordinary light (ne) pass polarizer in which a Ti-diffused optical waveguide is formed on a Z-cut LN substrate, Prior Art Document 1 discloses a method of attaching a film-shaped polarizer to a connection portion between a chip and a fiber, or Prior Art Documents 2-5 disclose a method of forming a low refractive index film or a high refractive index film on a Ti-diffused optical waveguide.
However, the extraordinary light (ne) pass polarizer has the following disadvantages.
(1) Since the polarizer is attached between the chip and the fiber (waveguide), light loss is large. In addition, since the polarizer is attached to each chip, productivity is low.
(2) From the relationship between Vπ and the optical bandwidth, it is necessary to separately form a thin film of a polarizer and a thin film below the electrode, which results in low productivity. In addition, since a region for the polarizer is needed, the size of an LN chip increases.